When you read, you make use of the ambient radiation in the visible spectrum, but the Sun also provides plenty of invisible infrared and ultraviolet radiation. The Sun's infrared and ultraviolet are the most intense of the invisible ambient radiation, but nature provides other sources.

Not exactly ambient field devices, but close cousins, are RFID devices. These are like ambient field devices, but they differ in the fact that the external field is purposefully generated, typically at a fixed frequency. RFID devices are designed to harvest energy at that frequency to energize their electronics, which often include a transmitter.

Even I've dabbled in passive RF communications. This is figure four of my January 29, 2013, patent, "Modulated antenna for wireless communications." In this case, I'm modulating an ambient radio frequency field using a Sterba Curtain antenna. (US Patent No. 8,362,961, via Google Patents.)[3)]

"We can repurpose wireless signals that are already around us into both a source of power and a communication medium... It's hopefully going to have applications in a number of areas including wearable computing, smart homes and self-sustaining sensor networks."[5]

A block diagram of an example ambient backscatter device is shown in the figure.

One important application for such devices is for communication with structural health sensors embedded into structures. Such sensors could communicate the presence of a crack. Credit-card sized proof-of-concept devices were constructed and tested in the Seattle area at locations ranging from about a half mile to six and a half miles distant from a television tower (see photograph).[5]

These devices could communicate at a data rate of a kilobit per second over a range of 2.5 feet outdoors, where the ambient signals were stronger, and 1.5 feet indoors, where the ambient signals were weaker.[5] This was more than enough to light a light-emitting diode associated with a particular button press and transfer "funds" from one credit card to another, as shown in a YouTube video.[5-6]